How to write an equation from a graph

 It depends what type of equation you are trying to solve for. 
For slope intercept form (y=mx+b)
 - pick two coordinates and find the slope using the slope formula; y2-y1 over x2-x1. 
- the y-intercept is usually found in (0, b) form. 
After you have found the slope and the y - intercept you can finally plug in your numbers to match the formula. 
Here is an example:
- My two coordinates on the line are (0,8) and (-1,5) 
- 8-5 over 0--1 = 3 over 1, or 3 is my slope.
- To find the y intercept look for the pair that is found in (0,b) form which in this case is (0,8) so my y intercept is 8.
You found your slope and y intercept so your equation is y = 3x + 8 
Hopefully this helped! This is how you find an equation for slope intercept form! If it is any other formula it is basically the same general steps.